This invention relates to drilling fluid additives and specifically surfactants which suppress the bit balling and sticking of the drill cuttings suspended in drilling fluids to the drillstring and drill pipe in a subterranean well during the drilling process. The invention is particularly directed to anti-bit bailing and lubricating materials comprising sodium alkylsulfosuccinates which are also environmentally acceptable.
In rotary drilling of subterranean wells numerous functions and characteristics are expected of a drilling fluid. A drilling fluid should circulate throughout the well and carry cuttings from beneath the bit, transport the cuttings up the annulus, and allow their separation at the surface. At the same time, the drilling fluid is expected to cool and clean the drill bit, reduce friction between the drill string and the sides of the hole, and maintain stability in the borehole's uncased sections. The drilling fluid should also form a thin, low-permeability filter cake that seals openings in formations penetrated by the bit and act to reduce the unwanted influx of formation fluids from permeable rocks. Finally, the drilling fluid is used to collect and interpret information available from drill cuttings, cores, and electrical logs.
Drilling fluids are typically classified according to their base material or primary continuous phase. In oil-base fluids, solid particles are suspended in oil, and water or brine may be emulsified in the oil. The oil is typically the continuous phase. In water-base fluids, solid particles are suspended in water or brine, and oil may be emulsified in the water. Water is the continuous phase. Environmental concerns have limited the use of oil-base drilling fluids. Accordingly, oil drilling companies have increasingly focused on water-base fluids.
The additive of this invention is primarily classified as a surfactant. Surfactants are used in all types of drilling fluids, and for several purposes. They serve as emulsifiers, foamers, defoamers, wetting agents, detergents, lubricants, and corrosion inhibitors. Surfactants are important constituents of both oil based muds and water based muds.
Current drilling fluid components must be environmentally acceptable. As drilling operations impact on plant and animal life, drilling fluid additives should have low toxicity levels and should be easy to handle and to use to minimize the dangers of environmental pollution and harm to personnel. Moreover, in the oil and gas industry today, it is desirable that additives work both onshore and offshore and in fresh and salt water environments.
It has been suggested that certain materials be added to a drilling mud to enhance the lubricating properties thereof. There are several objectives sought to be obtained by such addition. First, a conventional rolling cutter bit depends upon the drilling mud to lubricate the bearings of the cutters. It is not at all uncommon that a cutter bearing will fail before the cutter teeth have been worn out; and by prolonging the life of the bearings through increased lubrication, a longer bit life should be obtained. Such longer life would reduce the number of round trips required to change bits, reduce fishing jobs for lost cutters, and also reduce the danger of losing a hole during a round trip. Further, a drilling fluid having enhanced lubricating properties would reduce drill string torque, thereby reducing the likelihood of twist-offs. It would also permit the use of higher bit loadings and rotating speeds to increase the drilling rate. Moreover, it would be desirable for the lubricating properties of the drilling fluid to be such that the drill string would be oil wetted to minimize differential pressure sticking.
While other advantages might be mentioned, the above are sufficient to indicate the value of increasing the lubricating properties of drilling fluids.
In attempting to achieve the above advantages by the use of a lubricating or extreme-pressure additive to a drilling fluid, it has been found that the various known additives have been rather specific in their reaction in the drilling fluid. Thus, while a particular additive will work well in one environment, it may fail in another. For example, the additives heretofore suggested are sensitive to caustic so that they are either ineffective or of greatly reduced efficiency in fluids of elevated pH. Also, many are sensitive to oils so that their efficiency decreases as the oil content of the well fluid increases. Some of the additives have had undesired interaction with solids suspended in the drilling fluid. For example, they may cause the solids to be oil wetted. Some of the additives have poor compatibility with fluids containing substantial amounts of dissolved alkaline earth compounds such as lime base muds, wherein they tend to form curds. Foaming and poor dispersability have been the basis for objections to still other additives. Still other additives cause the drilling fluid to fluoresce under ultraviolet light which is objectionable, since such fluorescence interferes with "black light" well logging operations, particularly in wild cat wells.
It is apparent to anyone selecting or using a drilling fluid for oil and gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. As stated hereinabove, the typical compositions include oil based muds, water based muds and pneumatic fluids. For purposes of this application, only oil and water based mud systems will be relevant. The vast majority of oil and gas exploration is done with water based muds. The primary reason for this preference is price and environmental compatibility. Traditional oil based muds made from diesel or mineral oils, while being substantially more expensive than water based drilling fluids, are environmentally incompatible. As a result, the use of oil based muds has been historically limited to those situations where they are necessary.
The effectiveness of a drilling fluid and in particular the additives found in the drilling fluid are evaluated by measurement of certain characteristics of the drilling system. The viscosity, gel strength, filtrate loss, contamination control and tolerance to divalent ion characteristics of drilling fluids and drilling systems are all directly attributable to the components of the drilling fluid or drilling mud. These properties, their definitions and a general explanation is found in a comprehensive treatise entitled Composition and Properties of Drilling and Completion Fluids, 5th Ed., George R. Gray and H. D. H. Darley, Gulf Publishing Company, (1988).
Oil and water base muds have contrasting attributes and disadvantages. Although drilling fluids utilizing an oil based mud inhibit well bore swelling by minimizing dispersion fluid, the environmental toxicity of oil muds often overshadow the positive features. Oil base systems can be created with low toxicity but all systems are pollutants to varying degrees. In addition, the cost parameters of an oil mud are often prohibitive when compared to a water based system. Furthermore, the rheological and thixotropic character of an oil mud is not as versatile for maximized hole cleaning as certain aqueous fluids.
Cost effectiveness and environmental acceptability are major advantages of an aqueous drilling fluid but one major disadvantage also exists. Swelling and/or dispersion of formation clays causes general instability of the wellbore with resultant hole enlargement.
Inhibitive materials such as lime, gypsum, tannates and tannate/chrome lignosulfonate systems are used to minimize the factors causing an unstable wellbore.
Due to their chemical nature, these materials require frequent maintenance treatments and/or sufficient sodium hydroxide for solubilization. The resultant hydroxide radical ion causes the dispersion of the clay material. The degree of dispersion is proportional to the hydroxide radical concentration. The dispersion is a major cause of the high solids content of tannate and chrome lignosulfonate systems. A high quantity of low gravity solids content has detrimental effects on another parameter also. There is direct correlation between high quantities of low gravity solids content and reduced rates of penetration.